Heretofore, permanent connectors for optical fibers using glass have commonly had a V-shaped groove or a like construction for optical fiber alignment (centering).
Recently, a permanent connector for optical fibers including a glass sleeve in which optical fibers are concentrically butted against each other has been suggested (see Japanese Utility Model Application Disclosure Nos. 61505/1986 and 99806/1986).
In the case where optical fibers are to be butted against each other, the use of a V-shaped groove has a possibility of the ends of optical fibers deviating to the space opposite to the groove, and if this really happens, the connection loss increases.
The conventional permanent connector for optical fibers for concentrically butting optical fibers against each other in a glass sleeve uses a specially shaped single sleeve of nonuniform thickness in order that optical fibers which are each composed of a core having a thickness of about 10 .mu.m or about 50 .mu.m and a clad having an outer diameter of about 125 .mu.m concentrically surrounding said core and which are each covered with a coat having an outer diameter of about 250 .mu.m or about 900 .mu.m may be butted against each other. Manufacture of glass sleeves of such shape is very difficult because of their small diameter. And they are low in strength and their nonuniform thickness tends to cause stress concentration and they have no reinforcing construction. Therefore, they have been of little practical use.